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Exogenous antigens are antigens that have entered the body from the outside, for example by inhalation, ingestion, or injection.
The immune system's response to exogenous antigens is often subclinical.
By endocytosis or phagocytosis, exogenous antigens are taken into the antigen-presenting cells ( APCs ) and processed into fragments.
APCs then present the fragments to T helper cells ( CD4 < sup >+</ sup >) by the use of class II histocompatibility molecules on their surface.
Some T cells are specific for the peptide: MHC complex.
They become activated and start to secrete cytokines.
Cytokines are substances that can activate cytotoxic T lymphocytes ( CTL ), antibody-secreting B cells, macrophages, and other particles.

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